Over the last few updates I've noticed significant variation over the amount of HP the training dummy seems to have. (No, no -- before someone tries to bring up the helpless bit, it's not what I'm hitting him for, it's how much damage he takes.)
In update 14p2, 1241 points of sonic damage left him with a nearly invisible sliver of HP left.
In update 15, 1663 points of sonic damage left him with nearly two-thirds of his HP left.
There was some variation in the updates/patches prior to those two but I never recorded my observations.
So what's the story Turbine? Obviously I was using a high level toon, but my low level toons liked it alot more when he had around 1300hp.